Strength reduction compiler optimizations for operations with unknown strides

ABSTRACT

An optimizing compiler includes a strength reduction mechanism that optimizes a computer program that includes operations that have an unknown stride by analyzing the instructions in the computer program in a single pass, determining whether instruction substitution is profitable for original instructions in the code, and performing instruction substitution for one or more original instructions for which instruction substitution is deemed profitable, including operations with unknown strides. The substituted instructions result in strength reduction in the computer program.

BACKGROUND

1. Technical Field

This disclosure generally relates to computer systems, and morespecifically relates to compilers that generate executable code forcomputer systems.

2. Background Art

Computer systems have evolved into extremely sophisticated devices, andcomputer systems may be found in many different settings. Dramaticadvances in both hardware and software (e.g., computer programs) havedrastically improved the performance of computer systems. Modernsoftware has become very complex when compared to early computerprograms. Many modern computer programs have tens or hundreds ofthousands of instructions. The execution time (and hence, performance)of a computer program is very closely related to the number andcomplexity of instructions that are executed as the computer programruns. Thus, as the size and complexity of computer programs increase,the execution time of the computer program increases as well.

Unlike early computer programs, modern computer programs are typicallywritten in a high-level language that is easy to understand by a humanprogrammer. Special software tools known as compilers take thehuman-readable form of a computer program, known as “source code”, andconvert it into “machine code” or “object code” instructions that may beexecuted by a computer system. Because a compiler generates the streamof machine code instructions that are eventually executed on a computersystem, the manner in which the compiler converts the source code toobject code affects the execution time of the computer program.

The execution time of a computer program, especially complex computerprograms, is a function of the arrangement and type of instructionswithin the computer program. Many different optimizations have beendeveloped so the code produced by compilers has better run-timeperformance. One such optimization is known as “strength reduction”,which relates to replacing one or more expensive instructions with oneor more less-expensive instructions. The most common such optimizationis to replace repeated multiplications by repeated additions orsubtractions. Indeed, this is often the only form of strength reductionperformed by an optimizing compiler, since other related opportunities,such as divisions and modulos, are comparatively infrequent.

BRIEF SUMMARY

An optimizing compiler includes a strength reduction mechanism thatoptimizes a computer program that includes operations that have anunknown stride by analyzing the instructions in the computer program ina single pass, determining whether instruction substitution isprofitable for original instructions in the code, and performinginstruction substitution for one or more original instructions for whichinstruction substitution is deemed profitable, including operations withunknown strides. The substituted instructions result in strengthreduction in the computer program. One detailed method for the strengthreduction mechanism includes the steps of traversing the instructions inthe computer program in a single pass to build a candidate table,generating one or more increment tables from the candidate table,computing the cost of each increment in the increment table(s), andperforming instruction substitution of one or more alternativeinstructions for one or more original instructions that have a cost thatindicates instruction substitution is profitable, and therefore resultsin strength reduction. The strength reduction mechanism performsstrength reduction for known strides, and additionally performs strengthreduction for unknown strides.

The foregoing and other features and advantages will be apparent fromthe following more particular description, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The disclosure will be described in conjunction with the appendeddrawings, where like designations denote like elements, and:

FIG. 1 is a block diagram of an apparatus that includes a strengthreduction mechanism in an optimizing compiler;

FIG. 2 is a diagram showing a first sample snippet of code;

FIG. 3 is a diagram showing a prior art method for strength reductionfor the code shown in FIG. 2;

FIG. 4 is a diagram showing a second sample snippet of code thatcontains an unknown stride;

FIG. 5 is a diagram showing a strength-reduced snippet of codeequivalent to the code in FIG. 4;

FIG. 6 is a diagram showing a third sample snippet of code;

FIG. 7 is a diagram showing an unprofitable strength-reduced snippet ofcode equivalent to the code shown in FIG. 6;

FIG. 8 is a flow diagram of a method for performing strength reductionin a single pass for unknown strides;

FIG. 9 is a flow diagram of a method showing one suitable implementationfor method 800 in FIG. 8;

FIG. 10 is a diagram of a portion of a control flow graph of samplecode;

FIG. 11 is a diagram of a dominator tree for the code shown in FIG. 10;

FIG. 12 is a block diagram of a candidate table after processing thefirst instruction in block A;

FIG. 13 is block diagram of the candidate table after processing thesecond instruction in block A;

FIG. 14 is a block diagram of the candidate table after processing thefirst instruction in block B;

FIG. 15 is a block diagram of the candidate table after processing thesecond instruction in block B;

FIG. 16 is a block diagram of the candidate table after processing thefirst instruction in block E;

FIG. 17 is a block diagram of the candidate table after processing thesecond instruction in block E;

FIG. 18 is a block diagram of the candidate table after processing thefirst instruction in block C;

FIG. 19 is a block diagram of the candidate table after processing thesecond instruction in block C;

FIG. 20 is an increment table created from the candidate table shown inFIG. 19;

FIG. 21 is a cost table for the increment table in FIG. 20; and

FIG. 22 is a control flow graph of the code in FIG. 10 after performingthe strength reduction disclosed and claimed herein.

DETAILED DESCRIPTION

There are known methods for performing strength reduction on operationswith known strides in an optimizing compiler using multiple-passanalysis or loop analysis. To illustrate, consider the C language codesnippet in FIG. 2. We assume for this example a double is implementedusing eight bytes. The statement x+=a[j] multiplies j by the size of thedouble, namely 8, then adds this to a's address to find the address ofthe next array element. Thus, the statement x+=a[j] is equivalent to*(&a+(j*8)). The recurring value of 8 is called the “stride” of theoperation. Thus, a multiply (or shift) and an add are required in eachloop iteration to produce the address of the next array element.

A known optimizing compiler could perform strength reduction byreplacing the code in FIG. 2 with the code in FIG. 3. This is possiblebecause the stride, which is 8 in this specific example, is known.Strength reduction is most commonly done in known optimizing compilersfor code that includes loops, as shown in FIG. 2.

Known optimizing compilers do not perform strength reduction when thestride is unknown at compile-time. For the simple example above, theterm “stride” denotes the distance between an array element and the next(8 in the above example). As used herein, the term “stride” isgeneralized so any two operations containing multiplications c*s and d*shave a common stride s. The stride may be a constant (known) or avariable (unknown). This disclosure and claims focus on the case whenthe stride is unknown, meaning s is a variable and c and d areconstants. Because s is a variable, s is unknown at compile-time.

A simple example will illustrate limitations in prior art optimizingcompilers. Another sample code snippet is shown in FIG. 4. While thecode could be rewritten as shown in FIG. 5, because the stride “s” isunknown at compile-time, known optimizing compilers will not performstrength reduction on the code in FIG. 4. However, the code in FIG. 4could benefit from the strength reduction shown in FIG. 5. An optimizingcompiler will generate efficient code for these statements as follows.a=x+(3*s) will be replaced by a=x+(s<<2)−s; b=x+(5*s) will be replacedby b=x+(s<<2)+s; and c=x+(7*s) will be replaced by c=x+(s<<3)−s. Sincethe value of s<<2 need only be computed once, the cost of executing theoperation comprising the three statements in FIG. 4 is two shifts, fouradds, and two subtracts. Note, however, the introduction of a temporaryvariable can reduce the number of operations and give new opportunitiesfor strength reduction. Such a temporary variable “t” is shown in FIG.5. The first statement a=x+(3*s) can be replaced by a=x+(s<<1)+1. Thesecond statement t=s*2 requires one shift, which can reuse s<<1 from thefirst instruction. The statements b=a+t and c=b+t require only one addeach. The code in FIG. 5 is thus equivalent to the code in FIG. 4, yetonly uses 1 shift and 4 adds, five total steps, while the code in FIG. 5uses two shifts, four adds, and two subtracts, eight total steps. Thecode in FIG. 5 thus results in a savings of 37.5%, which would yieldincreased performance compared to the code in FIG. 4. However, becausethe stride “s” is unknown at compile-time, prior art optimizingcompilers would not perform any strength reduction on the code shown inFIG. 4. This simple example illustrates shortcomings in known optimizingcompilers, which do not perform strength reduction when the stride isunknown.

Another example is shown in FIGS. 6 and 7 to show that adding atemporary variable does not necessarily result in a reduction ofoperations. The code in FIG. 6 can be implemented using two shifts andtwo adds. The code in FIG. 7, after introduction of the temporaryvariable t, results in three shifts and two adds. Because theintroduction of the temporary variable t does not improve performance,an optimizing compiler needs to take into account cost of the code withthe temporary variable compared to cost of the original code. Theoptimizing compiler disclosed herein includes a strength reductionmechanism that analyzes a computer program in a single pass and computescost of the original code and cost of the new code to determine wheninstruction substitution may be performed in a manner that producesstrength reduction.

The claims and disclosure herein provide an optimizing compiler with astrength reduction mechanism that performs a single pass analysis of thecomputer program followed by determining where instruction substitutionis profitable. When profitable, the instruction substitution isperformed, including operations with unknown strides, resulting instrength reduction in the computer program.

Referring to FIG. 1, a computer system 100 is one suitableimplementation of a server computer system that includes an optimizingcompiler with a strength reduction mechanism that performs strengthreduction for operations with strides that are not known atcompile-time. Server computer system 100 is an IBM zEnterprise Systemcomputer system. However, those skilled in the art will appreciate thatthe disclosure herein applies equally to any computer system, regardlessof whether the computer system is a complicated multi-user computingapparatus, a single user workstation, or an embedded control system. Asshown in FIG. 1, computer system 100 comprises one or more processors110, a main memory 120, a mass storage interface 130, a displayinterface 140, and a network interface 150. These system components areinterconnected through the use of a system bus 160. Mass storageinterface 130 is used to connect mass storage devices, such as localmass storage device 155, to computer system 100. One specific type oflocal mass storage device 155 is a readable and writable CD-RW drive,which may store data to and read data from a CD-RW 195.

Main memory 120 preferably contains data 121, an operating system 122,source code 123, an intermediate representation 124, a compiler 125, andmachine code 127. Data 121 represents any data that serves as input toor output from any program in computer system 100. Operating system 122is a multitasking operating system. There are three differentrepresentations of a computer program in FIG. 1, namely the high-levelsource code 123, the intermediate representation 124 that is generatedby a front-end compiler from the source code 123, and the machine code127 that is generated by a back-end compiler from the intermediaterepresentation 124. The compiler 125 is preferably an optimizingback-end compiler that compiles the intermediate representation 124 andgenerates the machine code 127. Compiler 125 may also be used togenerate the intermediate representation 124 from the source code 123,or this may be done using a different compiler. The compiler 125includes a strength reduction mechanism 126 that analyzes the computerprogram in a single pass, and makes one or more instructionsubstitutions to improve the performance of the computer program, evenwhen the stride is unknown at compile-time. In the examples herein, thecompiler 125 operates on the intermediate representation 124 of thecomputer program. However, the compiler 125 could also operate on thesource code 123 as well.

Note the source code 123, intermediate representation 124, compiler 125,and machine code 127 are all shown residing in memory 120 for theconvenience of showing all of these elements in one drawing. One skilledin the art will appreciate that this is not the normal mode of operationfor most compilers. A front-end compiler processes source code 123 andgenerates therefrom intermediate representation 124. This processing mayoccur on a computer system separate from computer system 100. Compiler125 processes intermediate representation 124 and generates therefrommachine code 127, which may also occur on a separate computer system. Inthe extreme, source code 123 could reside on a first computer system anda front-end compiler could reside on a second computer system. Thefront-end compiler could read the source code 123 from the firstcomputer system, generate the intermediate representation 124, and storethe intermediate representation 124 on a third computer system. Compiler125 could be executed on a fourth computer system, which reads theintermediate representation 124 from the third computer system, andgenerates therefrom machine code 127, which could be written to a fifthcomputer system. This simple example shows that the preferredembodiments expressly extend to any suitable configuration and number ofcomputer systems to accomplish the front-end and back-end compiling. The“apparatus” described herein and in the claims expressly extends to amultiple computer configuration, as described by the example above.

Computer system 100 utilizes well known virtual addressing mechanismsthat allow the programs of computer system 100 to behave as if they onlyhave access to a large, contiguous address space instead of access tomultiple, smaller storage entities such as main memory 120 and localmass storage device 155. Therefore, while data 121, operating system122, source code 123, intermediate representation 124, compiler 125, andmachine code 127 are shown to reside in main memory 120, those skilledin the art will recognize that these items are not necessarily allcompletely contained in main memory 120 at the same time. It should alsobe noted that the term “memory” is used herein generically to refer tothe entire virtual memory of computer system 100, and may include thevirtual memory of other computer systems coupled to computer system 100.

Processor 110 may be constructed from one or more microprocessors and/orintegrated circuits. Processor 110 executes program instructions storedin main memory 120. Main memory 120 stores programs and data thatprocessor 110 may access. When computer system 100 starts up, processor110 initially executes the program instructions that make up operatingsystem 122. Processor 110 also executes the compiler 125.

Although computer system 100 is shown to contain only a single processorand a single system bus, those skilled in the art will appreciate thatan optimizing compiler as taught herein may be practiced using acomputer system that has multiple processors and/or multiple buses. Inaddition, the interfaces that are used preferably each include separate,fully programmed microprocessors that are used to off-loadcompute-intensive processing from processor 110. However, those skilledin the art will appreciate that these functions may be performed usingI/O adapters as well.

Display interface 140 is used to directly connect one or more displays165 to computer system 100. These displays 165, which may benon-intelligent (i.e., dumb) terminals or fully programmableworkstations, are used to provide system administrators and users theability to communicate with computer system 100. Note, however, thatwhile display interface 140 is provided to support communication withone or more displays 165, computer system 100 does not necessarilyrequire a display 165, because all needed interaction with users andother processes may occur via network interface 150.

Network interface 150 is used to connect computer system 100 to othercomputer systems or workstations 175 via network 170. Network interface150 broadly represents any suitable way to interconnect electronicdevices, regardless of whether the network 170 comprises present-dayanalog and/or digital techniques or via some networking mechanism of thefuture. Network interface 150 preferably includes a combination ofhardware and software that allow communicating on the network 170.Software in the network interface 150 preferably includes acommunication manager that manages communication with other computersystems 175 via network 170 using a suitable network protocol. Manydifferent network protocols can be used to implement a network. Theseprotocols are specialized computer programs that allow computers tocommunicate across a network. TCP/IP (Transmission ControlProtocol/Internet Protocol) is an example of a suitable network protocolthat may be used by the communication manager within the networkinterface 150.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wireline, optical fiber cable, RF, etc., or any suitable combination ofthe foregoing.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Smalltalk, C++ or the like and conventional proceduralprogramming languages, such as the “C” programming language or similarprogramming languages. The program code may execute entirely on theuser's computer, partly on the user's computer, as a stand-alonesoftware package, partly on the user's computer and partly on a remotecomputer or entirely on the remote computer or server. In the latterscenario, the remote computer may be connected to the user's computerthrough any type of network, including a local area network (LAN) or awide area network (WAN), or the connection may be made to an externalcomputer (for example, through the Internet using an Internet ServiceProvider).

Aspects of the present invention are described below with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

Referring to FIG. 8, a method 800 performs analysis of code in a singlepass and performs instruction substitution when profitable, resulting instrength reduction in the code. Method 800 is preferably performed bythe strength reduction mechanism 126 shown in FIG. 1. The code isanalyzed (step 810). When instruction substitution is profitable (step820=YES), instruction substitution is performed where profitable,including operations of unknown strides (step 830). The term “unknownstrides” is used herein to mean a stride that is unknown atcompile-time. As discussed above, prior art optimizing compilers do notperform strength reduction for operations with unknown strides. Method800, in contrast, performs strength reduction for operations withunknown strides as well as for operations with known strides.

Referring to FIG. 9, method 900 is one specific implementation formethod 800 in FIG. 8. Method 900 is also preferably performed by thestrength reduction mechanism 126 in FIG. 1. First, a control flow graphis generated (step 910). A dominator tree is generated for the controlflow graph (step 920). A candidate table of instructions is thengenerated, where the instructions in the candidate table will be in anorder determined by the dominator tree (step 930). One or more incrementtables are then generated from the candidate table. When a rootcandidate exists in the candidate table (step 932=YES), the next rootcandidate is selected (step 934). An increment table is then generatedby processing the root candidate and related candidates from thecandidate table (step 940). The cost of each increment in the incrementtable is computed (step 950). One of the increments in the incrementtable is selected (step 960). When the selected increment is notprofitable (step 970=NO), and there are more increments in the incrementtable to process (step 972=YES), method 900 loops back to step 960 andcontinues. When a selected increment is profitable (step 970=YES), aninitializer instruction for a temporary variable is inserted into thecode (step 974), one or more alternative instructions that reference thetemporary variable are substituted for one or more original instructions(step 980), and any unneeded instructions are removed (step 990).Instructions may be unneeded due to the substitution of the alternativeinstructions for the original instructions in step 980. When there aremore increments in the increment table to process (step 972=YES), method900 loops back to step 960 and continues. When there are no moreincrements in the increment table to process (step 972=NO), method 900loops back to step 932 and continues until all root candidates in thecandidate table have been processed (step 932=NO). At this point, method900 is done.

The steps in FIG. 9 are now illustrated using the specific example shownin FIGS. 10-22. Step 910 in FIG. 9 generates a control flow graph.Referring to FIG. 10, a control flow graph 1000 is shown that isrepresentative of a portion of a computer program. Control flow graphsare well-known in the art, and the various methods for constructing acontrol flow graph are also known. Note the sample instructions shown inFIG. 10 are extremely simplified for the purpose of illustrating theconcepts herein. In particular, many unrelated instructions, includingbranch instructions, have been omitted. The control flow graph 1000indicates that after executing the second instruction in basic block A,the program branches to either basic block B or basic block C.Similarly, after executing the second instruction in basic block B, theprogram branches to either basic block D or basic block E. The arrows inthe control flow graph represent possible flows between basic blocks.

Many known optimizing compilers use control flow graphs similar to thatshown in FIG. 10, but require multiple passes of analysis to performstrength reduction, or only perform strength reduction on loop inductionvariables. The disclosure and claims herein are directed to anapparatus, method and program product that include a strength reductionmechanism that performs a single pass analysis of a control flow graph,and that can operate on operations of unknown strides as well asoperations of known strides. In step 920 in FIG. 9, a dominator tree isconstructed. For the control flow graph in FIG. 10, the dominator tree1100 is shown in FIG. 11. A dominator tree may be constructed accordingto the concept of “dominance” in a control flow graph. Block A is saidto “dominate” block B, denoted “A dom B”, if every path from thebeginning of the procedure to B must pass through A. Thus, A is alwaysexecuted at least once before B is reached the first time. A is said tobe the unique “immediate dominator” of B, denoted “A idom B”, providedthat A dom B and there does not exist a block C such that A dom C and Cdom B. The immediate dominator relation induces a tree on the nodes ofthe control flow graph where the edge A→B exists in the tree if and onlyif A idom B. Every node in the control flow graph has an immediatedominator, so the dominator tree 1100 in FIG. 11 completely covers thenodes of the control flow graph 1000 in FIG. 10. Thus, a topologicalwalk of the tree in a single forward pass visits each basic block onlyafter all of its dominators have been visited.

With the above description of dominance in a control flow graph, thedominator tree 1100 in FIG. 11 may be constructed from the control flowgraph 1000 in FIG. 10. Dominator tree 1100 shows that block A is theimmediate dominator of blocks B, C, and F, and block B is the immediatedominator of blocks D and E. A dominates every block in the graph, whileblocks C, D, and E do not dominate any other blocks. Construction of thedominator tree 1100 is well known in the art, as shown in Lengauer etal., “A Fast Algorithm for Finding Dominators in a Flowgraph”, ACMTransactions on Programming Languages and Systems, Vol. 1, No. 1, pp.121-141, July 1979.

The instructions in the control flow graph are analyzed in an orderdetermined by the dominator tree, and instructions that may becandidates for strength reduction are placed into a candidate table. Thebasic blocks in the control flow graph may be visited in any order thatrespects the dominance ordering represented in the dominator tree, whichmeans a block is never visited prior to its immediate dominator. Twosuch orderings for the dominator tree 1100 in FIG. 11 are ABDECF andABCFDE. We will use ABDECF in this example. Within each block,statements are visited in forward order. Assume that x and s arevariables that are input to the program.

A candidate table may then be generated as shown in step 930 in FIG. 9.The candidate table is built one instruction at a time, and includes theinstructions in the program being analyzed. Referring to FIG. 12, thecandidate table is shown after analyzing the first candidate instructionin basic block A, namely a=5*s. The candidate table shown in FIGS. 12-19include columns labeled Stmt, Num, Base(B), Index(i), Stride(S), Kind,Next Interp, Basis, Dep, Sib and Dead Savings. These columns (or fields)of the candidate table shown in FIGS. 12-19 are described in more detailbelow with reference to the specific control flow graph in FIG. 10. Notethe expression C(*, 5) shown in the Dead Savings field of the second rowin FIG. 13 is shorthand for “the cost of multiplying a variable by theconstant 5.” The discussion below explains how the candidate table ispopulated one instruction at a time.

When each candidate instruction or statement is visited, a set of rulesare used to determine whether the statement refers to an expression ofthe form (B+i)*S, referred to here as a MULT, or of the form B+(i*S),referred to here as an ADD. B and S must be variables, and i must be aconstant. For example, the right-hand side of the first statement a=5*scan be written as (s+0)*5, with B=s, i=0, and S=5.

We assume that the program is in static single-assignment form, so thateach variable appears on the left-hand side of, i.e., is defined by, atmost, one statement. When encountering a variable V on the right-handside of a statement S whose definition already appears in the table, theknown values of B, i, and S for variable V are factored into the entryfor statement S. So for statement 2 (b=x+a), we combine b=x+a with a=5*sto get the expression x+(5*s), with B=x, i=5, and S=s. There may be manyrules for combining various statements, which are not all describedhere.

If, in the foregoing discussion, the variable V has no other uses thanthe one in statement S, the possibility exists that we will be able toremove the statement that defines V in a subsequent optimization. Toease later analysis, we record the cost of computing V in the “DeadSavings” field of S. For statement 2 in FIG. 13, this is the cost ofmultiplying s by the constant 5. The cost is estimated based on thecharacteristics of the target processor, and may be measured in anyuseful units, such as instructions or CPU cycles. Dead-code savings iscumulative, so if the candidate entry that defines V has a nonzero costin its Dead Savings field, that is added into C's Dead Savings field.Note that b=x+a has two variables on the right-hand side. There are twoways of interpreting this: either B=x, or B=a. That is, we can considerthe right-hand side to be x+(5*s), or a+(1*x). In the latter case, x isa program input, so it does not have a candidate entry in the table.When there are multiple interpretations for a statement, eachinterpretation gets a separate entry in the candidate table, and theyare linked together in a chain using the Next Interp field as describedin detail below.

Whenever an ADD candidate C is placed in the table, we check whether anyprevious entries in the table can be used as a basis for C. A previousADD candidate P can serve as a basis for C if P and C have identicalBase and Stride fields, and if P dominates C. Because we are processingblocks using an ordering that respects dominance, all statements thatcan dominate C are already in the candidate table.

Note that C may have more than one basis in the table. In this case, anypolicy may be used to select one of these as C's basis. In this example,we choose the basis that appears latest in the table, which is the “mostimmediately dominating basis”, but several other policies could also beused within the scope of the disclosure and claims herein.

If P is a basis for C, the reverse relationship is stated as “C is adependent of P.” Thus when a basis P is found for C, P's candidatenumber is recorded in C's Basis field, and C's candidate number isrecorded in P's Dependent field. If P already had a nonzero Dependentfield (because another candidate uses P as a basis), the previous valueof P's Dependent field is stored in C's Sibling field. Thus alldependents of P can be found by recursively following the Dependent andSibling fields of P's immediate dependent. With all this in mind, we cansee how the candidate table is created. As previously discussed, a=5*sis given a MULT entry in the table with B=s, i=0, and S=5. It has onlyone variable on the right-hand side, so it does not have an alternateinterpretation. Since it is not an ADD, all remaining fields are zero,as shown in FIG. 12.

The next candidate instruction b=x+a is given two ADD entries forx+(5*s) and a+(1*x). There is no previous entry with B=x and S=s, so thefirst interpretation does not have a basis. There is no previous entrywith B=a and S=x, so the second interpretation does not have a basis.Initially the Dependent and Sibling fields for both are set to zero.Because the first interpretation relies on the previous candidate a=5*s,and a does not have any other uses in the program, we place C(*, 5) inthe Dead Savings field, as shown in FIG. 13.

Candidate 4 (c=7*s) is processed exactly like candidate 1. which resultsin the candidate table shown in FIG. 14.

Candidates 5 and 6 (d=x+c) are processed similarly to instructions 2 and3. This time, however, candidate 5 has a basis in candidate 2, becauseboth candidates have B=x and S=s, and statement 2 dominates statement 5(block A dominates block B). So candidate 5's Basis field is set to 2,and candidate 2's Dependent field is set to 5, as shown by the bold 5 inthe Dep field of candidate 2 in FIG. 13. Since it was previously 0, noSibling processing is necessary. The resulting candidate table is shownin FIG. 15.

Candidate 7 (g=9*s) is processed exactly like candidates 1 and 4,resulting in the candidate table shown in FIG. 16.

Candidates 8 and 9 (h=x+g) are processed similarly to candidates 5 and6. This time candidate 8 has two possible bases in candidates 2 and 5.Since the selected policy is to choose the most immediate dominatingbasis, candidate 5 is chosen as the basis. Candidate 8's Basis field isset to 5, and candidate 5's Dependent field is set to 8, as shown inbold in FIG. 17. Since it was previously 0, again no Sibling processingis necessary. The resulting candidate table is shown in FIG. 17.

Candidate 10 (e=8*s) is processed exactly like candidates 1, 4, and 7,resulting in the candidate table shown in FIG. 18.

Candidates 11 and 12 (f=x+e) are processed similarly to candidates 2 and3. This time candidate 11 has only one basis which is candidate 2.Candidate 11's Basis field is set to 2, and candidate 2's Dependentfield is set to 11, as shown in bold in FIG. 19. However, sincecandidate 2's Dependent field was previously 5, now candidate 11'sSibling field is set to 5 so this dependency is not lost. The result isthe candidate table shown in FIG. 19, which includes all candidates fromthe control flow graph 1000 in FIG. 10.

Next, each set of related candidates in the candidate table isidentified, analyzed, and optimized as a group. Two candidates are“related” if a transitive walk of their Basis fields reaches the same“root” candidate. A root is a candidate that has no basis (Basis=0) andhas at least one dependent (Dependent≠0). The candidate table in FIG. 19is scanned from the top down to find the first such root candidate. Thenevery related candidate in the tree is processed recursively via theSibling and Dependent links to form an increment table for the set ofrelated candidates, as described below. Note that there is a separateincrement table for each set of related candidates.

FIG. 20 shows such an increment table. The Increment field contains avalue by which at least one candidate's index field differs from theindex field of its basis. The Count field indicates how many replaceablecandidates have this increment. If not zero, the Initializer fieldidentifies an existing variable that represents the expression I*S,where I is the value in the Increment field and S is the common Strideof all the related candidates. The Cost field represents the cost ofreplacing each statement having this increment with an addition relativeto its basis. If the value is less than zero, the replacement isprofitable. The generation of the increment table in FIG. 20 isdiscussed in detail below.

For each candidate to be processed, first calculate its increment. Forevery candidate C except the root, the increment is the differencebetween C's Index field and the Index field of C's basis. For example,statement 5's basis is statement 2, and their respective indices are 7and 5, so the increment for statement 5 is 7−5=2. For the root, wedefine the increment to just be the root's Index field.

There is only one root candidate in our example: Statement 2 has Basis 0and Dependent 11. As a root statement, its increment is the same as itsIndex field (5). We create an entry in the increment table for increment5. Because the root candidate will never itself be a candidate forreplacement, we set the count field to 0 and the cost field to infinity.Because the root statement provides the expression 5*s, and theIncrement field is 5, the left-hand side of statement 2 (a) is stored inthe Initializer field.

Following statement 2's Dependent field, we next process statement 11.Statement 11's increment is 8−5=3. There is no entry in the table forincrement 3, so a new entry is created with Increment=3 and Count=1. Noinitializer is available for 3*s. The cost field will be calculatedlater.

Following statement 11's Sibling field, we next process statement 5.Statement 5's increment is 7−5=2. There is no entry in the table forincrement 2, so a new entry is created with Increment=2 and Count=1. Noinitializer is available for 2*s.

Following statement 5's Dependent field, we next process statement 8.Statement 8's increment is 9−7=2. An existing entry for increment 2 isfound, so its Count field is incremented to 2.

Statement 8 has no Dependent or Sibling field, so we return to statement5. Statement 5 has no Sibling field, so we return to statement 11.Statement 11 has no Dependent field, so we return to statement 2.Statement 2 has no Sibling field, and is the root of the tree, so thegeneration of the increment table from the candidate table is complete.

Next, the cost for each increment is computed. For most increments I, aninitializer t=I*S will have to be inserted into the code, and then theright-hand side of each candidate other than the root will be replacedby t+b, where b is the value of the candidate's basis statement. Thecost of inserting the initializer is offset by the value of anystatements that can be removed when making the replacement, as stored inthe candidate's Dead Savings field. Note that increments 0, 1, and −1are cheaper to introduce, because the initializer need not be inserted.Instead, the right-hand side of each candidate is replaced by b, b+s, orb−s, respectively.

Looking at the example, we first look at the entry for increment 5.Since the count is zero, nothing is done with this increment.

For increment 3, the Count field is 1, and the single related candidateis statement 11. To replace statement 11, we must insert an initializert=3*s, at cost C(*, 3). From the candidate table, we see that thereplacement allows removal of some existing statements at a savings ofC(*, 8), so the total cost is C(*, 3)−C(*, 8).

For increment 2, the Count field is 2, and the two related candidatesare statements 5 and 8. To replace these, we must insert an initializert=2*s, at cost C(*, 2). From the candidate table, we see that replacingeach of these statements allows removal of some existing statements at asavings of C(*, 7) and C(*, 9), respectively. So the total cost is C(*,2)−C(*, 7)−C(*, 9).

Up to this point, we have treated costs such as C(*, n) symbolically.For any particular target processor, we must estimate concrete valuesfor these costs. On most modern processors, the cost of a multiplyinstruction is much higher than the cost of simpler instructions such asa left-shift or an addition. If the simpler instructions take a singlemachine cycle to execute, the multiply might take tens of machinecycles. So multiplies by a constant are usually replaced by a sequenceof shifts and/or adds and subtracts.

Typical sequences for multiplying a variable x by the values used inthis example are shown in the table in FIG. 21. We assume that each ofthese costs has a value of either 1 or 2 cycles. The cost for increment3 is then C(*, 3)−C(*, 8)=2−1=1. Since this is positive, the replacementis not profitable. The cost for increment 2 is C(*, 2)−C(*, 7)−C(*,9)=1−2−2=−3, so the replacement is profitable.

For each profitable increment, the next step is to insert an initializerif one is needed. An initializer is an instruction that defines atemporary variable that is inserted so the strength reduction may beperformed. The only profitable increment is 2, and there is noinitializer 2*s, so we must insert an initializer instruction t=2*s,where the variable t is the new temporary variable. Note that if such aninitializer already existed, we would not insert a new instruction, butwould instead use the variable defined by the initializer instead of anew variable t. We must find a location that dominates both candidates 5and 8. Referring to FIG. 10, these statements are in block B and E, andB dominates E, so we can place the initializer in block B provided weinsert the initializer prior to statement 5. So a statement t=2*s isinserted immediately prior to statement 5, as shown in basic block B inFIG. 22.

We can now replace statements 5 and 8 with statements that use theinitializer and the respective bases. Statement 5 is replaced by d=b+t,and statement 8 is replaced by h=d+t. The results computed by statements4 and 7 are no longer needed, so these statements are removed. In thealternative, removal of these instructions could be left to a separate“dead code elimination” pass that removes instructions whose producedvalues are no longer used. The resulting control flow graph is shown inFIG. 22. Because original instructions have been replaced withalternative instructions that have lower cost, the result is strengthreduction in the code.

The claims and disclosure herein provide an optimizing compiler thatincludes a strength reduction mechanism that optimizes a computerprogram that includes operations that have an unknown stride byanalyzing the instructions in the computer program in a single pass,determining whether instruction substitution is profitable for originalinstructions in the code, and performing instruction substitution forone or more original instructions for which instruction substitution isdeemed profitable, including operations with unknown strides. Thesubstituted instructions result in strength reduction in the computerprogram.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the claims. Thus, while the disclosure isparticularly shown and described above, it will be understood by thoseskilled in the art that these and other changes in form and details maybe made therein without departing from the spirit and scope of theclaims.

The invention claimed is:
 1. An apparatus comprising: at least oneprocessor; a memory coupled to the at least one processor; a computerprogram residing in the memory, the computer program including aplurality of instructions; and an optimizing compiler residing in thememory and executed by the at least one processor, the optimizingcompiler including a strength reduction mechanism that analyzes thecomputer program in a single pass by: constructing a control flow graphof the instructions in the computer program; building a dominator treecorresponding to the control flow graph that indicates which blocks inthe control flow graph dominate other blocks in the control flow graph;determining an order of analyzing the instructions in the control flowgraph in the single pass based on the dominator tree; constructing acandidate table of instructions in the computer program in an orderdetermined by the dominator tree; generating at least one incrementtable by processing the candidate table; computing cost of eachincrement in the increment table; determining whether a selectedincrement in the increment table is profitable based on the computedcost; for each selected increment in the increment table, when the costfor the selected increment indicates the selected increment isprofitable: inserting an initializer instruction for a temporaryvariable; and substituting at least one alternative instruction thatreferences the temporary variable for at least one of the plurality ofinstructions in the computer program; wherein the strength reductionmechanism determines whether an operation in the computer program thathas an unknown stride at compile-time could be improved by substitutingat least one alternative instruction for the at least one of theplurality of instructions in the operation, and when the operation inthe computer program that has an unknown stride at compile-time could beimproved, substituting the at least one alternative instruction for theat least one of the plurality of instructions, wherein the strengthreduction mechanism determines whether an operation in the computerprogram that has an unknown stride at compile-time could be improved bycomputing a number of operations to execute the at least one alternativeinstruction, computing a number of operations to execute the at leastone of the plurality of instructions, and when the a number ofoperations to execute the at least one alternative instruction is lessthan the a number of operations to execute the at least one of theplurality of instructions, determining the operation in the computerprogram could be improved.
 2. The apparatus of claim 1 wherein thestrength reduction mechanism further removes at least one unneededinstruction from the computer program, wherein the instruction isunneeded due to the insertion of the temporary variable and thereplacement of the at least one of the plurality of instructions withthe at least one alternative instruction.
 3. An article of manufacturecomprising software stored on a non-transitory computer readable storagemedium, the software comprising: a strength reduction mechanism thatanalyzes in a single pass a computer program that has a plurality ofinstructions by: constructing a control flow graph of the instructionsin the computer program; building a dominator tree corresponding to thecontrol flow graph that indicates which blocks in the control flow graphdominate other blocks in the control flow graph; determining an order ofanalyzing the instructions in the control flow graph in the single passbased on the dominator tree; constructing a candidate table ofinstructions in the computer program in an order determined by thedominator tree; generating at least one increment table by processingthe candidate table; computing cost of each increment in the incrementtable; determining whether a selected increment in the increment tableis profitable based on the computed cost; for each selected increment inthe increment table, when the cost for the selected increment indicatesthe selected increment is profitable: inserting an initializerinstruction for a temporary variable; and substituting at least onealternative instruction that references the temporary variable for atleast one of the plurality of instructions in the computer program;wherein the strength reduction mechanism determines whether an operationin the computer program that has an unknown stride at compile-time couldbe improved by substituting at least one alternative instruction for atleast one of the plurality of instructions in the operation, and whenthe operation in the computer program that has an unknown stride atcompile-time could be improved, substituting the at least onealternative instruction for the at least one of the plurality ofinstructions, wherein the strength reduction mechanism determineswhether an operation in the computer program that has an unknown strideat compile-time could be improved by computing a number of operations toexecute the at least one alternative instruction, computing a number ofoperations to execute the at least one of the plurality of instructions,and when the a number of operations to execute the at least onealternative instruction is less than the a number of operations toexecute the at least one of the plurality of instructions, determiningthe operation in the computer program could be improved.
 4. The articleof manufacture of claim 3, wherein the strength reduction mechanismfurther removes at least one unneeded instruction from the computerprogram, wherein the instruction is unneeded due to the insertion of thetemporary variable and the replacement of the at least one of theplurality of instructions with the at least one alternative instruction.